The principle of the power generation by fuel cells was found by the experiment which done by British judge, William Grove in 1839. A fuel cell is an electrochemical energy conversion device that converts hydrogen and oxygen into water, producing electricity and heat in the process. Unlike the ordinary non-rechargeable battery which are disposed after use, It can be recharged in order to retain its electrical power by adding fuels (e.g. hydrogen or methanol) continuously.
The current fuel cells can be classified by the type of electrolyte they use: alkaline fuel cells (AFC), phosphoric acid fuel cells(PAFC), molten carbonate fuel cells(MCFC), solid oxide fuel cells(SOFC) and proton exchange membrane fuel cells(PEMFC). Typical prior arts such as U.S. Pat. No. 5,272,017 with a title of “Membrane-electrode assemblies for electrochemical cells”, U.S. Pat. No. 5,316,871 with a title of “Method of making membrane-electrode assemblies for electrochemical cells and assemblies made thereby”, etc. Fuel cells can also be classified by the type of fuels they use, such as hydrogen-oxygen fuel cells, direct methanol fuel cells, hydrazine fuel cells, and zinc air fuel cells etc.
For example, direct methanol fuel cells (DFMC) use methanol aqueous solution as the fuel. When providing current, methanol is electrochemically oxidized at the anode electrocatalyst to produce electrons which travel through the external circuit to the cathode electrocatalyst where they are consumed together with oxygen in a reduction reaction and can recombine with the hydrogen ions and oxygen to generate by-product water. The proton exchange membranes mainly utilize polymeric electrolyte conducting ions. In the current literature, the common polymeric electrolytes were mainly fluoric polymeric structural grafting sulfonates, wherein the most widely used membrane in the current fuel cell systems are Nafion membranes (its structure shown in FIG. 1 and the micro-structure shown in FIG. 10).
The most difficult problem in the use of PEM in DMFC is that methanol and water are highly compatible molecules and can easily form complexes with protons. Protons are ions containing no electrons. As naked protons, protons have strong interactions with their environment to form complexes due to the absence of shielding of nuclear charges. If PEM needs to have higher proton conductivity, its chemical structure usually generate a strong hydrophilic environment and the hydrophilic environment made quite easy for methanol to be “hydrated”. Therefore, methanol fuels used in DMFC are easily brought through PEM by combining with protons on the anode of the cell and it results in the losses of fuels on the anode. At the same time, the catalysts and oxygen on the cathode are consumed and the catalysts on the cathode are poisoned, which reduce the activities of the electrodes. This phenomenon is called “methanol crossover”. This is one of the main causes that result in the poor efficiency of DMFC. The concentration of methanol fuels cannot be raised that also results in the decrease of the entire volume energy density of DMFC. Therefore in the research field of DMFC, the solution for the problem of over-permeable methanol in methanol fuels for PEM materials is the most critical technical bottleneck. The current solutions related to the permeability of methanol in materials are following:    1. To reduce the concentration of ion conducting groups (Ion Exchange Capacity, IEC) of PEM materials or to select other substitute materials for PEM (in stead of Nafion). The IEC value in PEM is the key to determine the proton conductibility of PEM. Higher IEC value will easily generate hydrophilic clusters in its structures, which reversely results in higher permeation of methanol. Therefore, some known technologies and researches utilize PEM with different IEC values to make the laminated structures or subject benzene-ring containing polymers to sulfonation in order to control the concentrations of ions in the system and to reduce the permeation of methanol. However, these technologies are mostly to be operated under high temperature for better proton conductivities, and most of the modification methods to theses membranes will result in the decrease of proton conductivities when reduced permeability of methanol. In addition, there are some literatures which use the conventional concepts of composite materials and blend electrolytes with other fiber or porous plastic material to reduce the swelling of the electrolytes by enforcing the materials. However, this also reduces the proton conductivities at the same time. The related prior arts are listed in Table 1.
TABLE 1Prior artsTechnical strategies and functionsU.S. Pat. No. 5,525,436Acid doped-PBI′ using the hetero-ring ofU.S. Pat. No. 5,716,727imidazole to provide proton conductibilityU.S. Pat. No. 6,025,085However, it is more suitable for high temper-U.S. Pat. No. 6,099,988ature and no-water environment condition.U.S. Pat. No. 6,124,060U.S. Pat. No. 5,599,639U.S. Pat. No. 6,365,294Use of polyphosphazene substrate as PEMU.S. Pat. No. 6,444,343PSSA + PVDF cross-linked membrane toobtain low crossover PEM.U.S. Pat. No. 4,626,553Laminated two different IEC values PEM withsame main chains of polymericU.S. Pat. No. 5,447,636Nafion + HDPE′ PTFE laminate to enhanceU.S. Pat. No. 5,795,668the selectivity and to increase the mechanicalproperties of membranesU.S. Pat. No. 6,242,122Laminated film′ one of the layers as PdmembraneU.S. Pat. No. 5,981,097Two or more layers of laminated film withU.S. Pat. No. 4,672,438different ion exchange capacitiesU.S. Pat. No. 5,654,109laminated film with different IEC values    2. To change the proton conducting mechanism of PEM materials. It is expected to change from the vehicle mechanism of protons in PEM to the proton hopping mechanism for conducting by using inorganic solid acid materials. Organic materials hardly have proton hopping characteristic whereas inorganic materials always have its innate limitation in film-forming process. Moreover, there are only limited inorganic materials with high proton conductibility at room temperature and most of them are soluble in water. Therefore, prior arts in this field have limited breakthrough and development. Known related patented technologies are listed Table 2.
TABLE 2Prior artsTechnical strategies and functionsU.S. Pat. No. 4,594,297PVA + heteropoly acid, apply in gas phase fuelfeedU.S. Pat. No. 4,380,575heteropoly acid solid electrolyte′ apply in gasphase fuel feedWO 9852243Completely apply zeolite as the electrolyte    3. Organic/inorganic blended materials—to reduce the hydrophilic cluster volume of conventional PEM materials. In the early researches of PEM, in order to increase the water saturation of PEM used at high temperature or to reduce the crossover phenomenon of hydrogen/oxygen gases, some of prior arts use simple synthetic reactions to fill inorganic metal oxides into the cluster of PEM materials or to blend them directly with PEM materials. According to this approach, it expects to enhance the proton conductibility of PEM at high temperature and to reduce the permeation of fuels, for example, to reduce the permeation of methanol in DMFC. However, most of the experimental results are found that this hardly have significant improvements. The reason is that the proton conductibility decreases due to the reduction of the conductive pathways with the decreases of cluster volume in PEM materials while the permeability of certain methanol in PEM is reduced by using inorganic materials. These known patented technologies are listed Table 3 below.
TABLE 3Prior artsTechnical strategies and functionsU.S. Pat. No. 4,687,715PTFE + ZrOP porous membranes added intoconductorsU.S. Pat. No. 5,849,428New method of depositing ZrOP in PEMU.S. Pat. No. 5,919,583PEM + inorganic proton conductors used inDMFC result in low fuel crossover and highconductivityU.S. Pat. No. 6,059,943PEM + ZrOP used in high temperature andU.S. Pat. No. 6,387,230enhancing the conductivity, SOL-GEL methodU.S. Pat. No. 5,795,496s-PEEK, s-PES + zeolite, by using sulfonationof high performance engineering plastics tomake the polymer membrane with protonconductivity. Additionally adding zeolite toreduce crossoverU.S. Pat. No. 6,447,943PEM + Porous + Acid, by using porousmaterials to create spaces for filling in liquidor solid acids to reduce crossover
In addition, from the operation of DMFC perspective, it should recover the by-product water of the cathodic reaction from the system if needed to efficiently enhance the energy density of the materials in the future since the theoretical reaction concentration ratio of methanol to water is 1:1 molar ratio. The water could be mixed with high concentration of fuels on the anode to increase the energy density of fuels. The water management of fuel cells in the previous hydrogen-oxygen system is the focus of researches since the hydrogen-oxygen system fuel cells are operated at the temperature higher than 100° C. Therefore water in PEM materials is quite easily evaporated to dryness due to high temperature. Consequently, the system utilizes the water recovered from the cathode to increase the wetness, such as shown in U.S. Pat. No. 4,769,297. In DMFC systems, current DMFC systems focus on the application of the portable products. The key of main technologies is how to effectively enhance the fuel converting efficiency of the materials (the amount of power density and fuel utilization rate). Currently the important issues of DMFC to be operated at room temperature can be classified as following: 1. the permeability of methanol fuels is too high which results in low fuel utilization rate and can not increase the volume energy density of the system. 2. the recovery of the water by-product on the cathode. If the water formed by the reaction on the cathode could not be removed smoothly, it would block catalysts of the cathode which results in insufficient oxygen concentrations and decreased utilization rate of catalysts. Therefore, a good design of water and heating management mechanism is essential to ensure the fuel cells have a high performance and high energy efficiency. Due to the use of additional recovery apparatus such as pumps for the hydrogen-oxygen system in the past, its design is a technology to install a water-collecting apparatus on the cathode and to flow back the water collected by pumps for the use of the anode, for example in U.S. Pat. Nos. 4,037,024, 4,826,741, 6,117,577, 6,432,568 and 6,579,637. Alternatively, it can utilize the pressure controls of the two ends in the feed materials to recover water of the cathode, for example in U.S. Pat. Nos. 5,503,944, 5,700,595, 5,853,909 and 6,586,128. As above, if it applied to miniaturized DMFC systems, the volume and complexity of cell systems are increased and it needs extra electric power to drive the pump. The current technology for resolving water management problems in DMFC systems is mainly adding more hydrophobic materials (such as PTFE) on the electrodes to repel water back to the anode. Nevertheless the effect of this approach is limited and the electrolytes and membranes can easily delaminate after the long use of the cells. The related approaches are in U.S. Pat. Nos. 6,277,513, 6,458,479, 6,492,052, 6,509,112 and 6,596,422.